Isotope detectives

Measuring bones and teeth can tell us a lot, but looking at them at the atomic level can tell us more. Your tooth enamel and bones contain a lot of calcium and also very small amounts of strontium, Sr.
Strontium comes from the rocks beneath your feet. Over thousands of years the strontium is taken up by plants and animals that feed on the plants. When you eat the plants and animals it gets used alongside calcium to build tooth enamel and bone.

Strontium has four stable isotopes. Two of these are 86Sr and 87Sr. The ratio of 87Sr to 86Sr in the environment depends on the age of the rocks beneath the soil. Fig. 4.1 shows a map of strontium 87/86 ratios.
You can find the map here and a link to the map on Google Earth http://www.bgs.ac.uk/nigl/SBA_Maps.htm

Fig. 4.1 Strontium levels map showing Fishbourne

The map shows the different strontium 87/86 ratios found in the geology of the Britain. These Sr values transfer through the food chain to leave a signature in skeletal material. This means an animal will have the same Sr value as the geology they live on. Teeth are especially useful to analyse, because when they are forming they record the geology of the local area. This allow us to identify if individuals have moved geological location

When you get your adult teeth, the enamel stays with you for life. Bone
is different. It is replaced several times during your life as cells are
lost and replaced. This means that strontium in your teeth can tell you
where you grew up and strontium in your bones can tell where you have
lived for the last 10 or 20 years. The same is true for animals.

Fallow deer jawbones found at Fishbourne revealed something about their life history.

Useful video - The Fallow Deer of Fishbourne Roman Palace

This video examines the circumstances and cultural significance of this species' diffusion across Europe.

Fig. 4.2 Fallow deer jawbone from AD 60 found at Fishbourne.

Molar M1 developed before M2 and M3, so Sr ratios for M1 will tell us about where the deer was when it was young and ratios for M2 where it was a few years older.

Here are the strontium 87/86 ratios for the teeth of deer and other animals used for comparison.

Fig. 4.3 Strontium 87/86 ratios for deer teeth found at Fishbourne.

For the AD 60 deer, its late forming teeth (M2 and M3) plot in the green area, with the Fishbourne animals. However, the animal’s M1 indicates that it was born in an area of very different geology. This means that the AD 60 deer was brought to the Palace at a young age. This could be one of the first fallow deer ever to have been brought to Britain as the Romans invaded Britain in AD 43.

Isotope evidence from Fishbourne is supported by the literature of the time. For much earlier periods there is no written history. For these we have to rely on isotopes to show how animals may have been moved.

Traveling Cows in 2500 BC

Figure 4.4 shows strontium isotope evidence from about 15 teeth that cows found at Durrington Walls, a Neolithic village near Stonehenge, may have been raised in Scotland. Locally reared cows would show different strontium 87/86 ratios that are found on chalk land. The evidence that Durrington Walls was a site for feasting and that many animals were slaughtered there. Isotope evidence seems to show that some of the cows came great distances.

Questions

1. Find calcium and strontium on a periodic table. Are they in the same group?

2. How many protons are there in strontium 86 and strontium 87 atoms?

3. How many neutrons are there in strontium 86 and 87 atoms?

4. Can evidence from a a few teeth be used to explain why cows in Durrington Walls came from Scotland?

5. Why is the strontium 87/86 ratio in tooth enamel a less reliable indicator of geographical origin for humans today?